The RTM process has been used for many years in the production of integral components. It is a resin infusion process which is usually used for producing fiber composite components. According to Wikipedia, resin transfer molding (RTM) is a process for producing molded parts from duroplastics and elastomers. In comparison to pressing, the molding compound is injected by means of a piston from a usually heated prechamber via distributor channels into the mold cavity, where it cures under heat and pressure.
Formaldehyde resins (PF, MF, etc.) and reaction resins (UP, EP) containing small filler particles and elastomers may be used as molding compound.
At the start of a cycle, a preplasticized, metered molding compound is present in a prechamber. First, the mold is closed. The molding compound is subsequently injected into the mold and left therein for a certain time. During this so-called residence time, the molding compound reacts or vulcanizes. This depends on various factors (type of resin, filler, process pressure and temperature). The mold may be opened when the residence time has ended. The previously filled molding compound is now solid (cured), and is referred to as a molded part. The molded part may now be removed from the mold. The mold is subsequently cleaned, and a new cycle may begin. The dimensions of the molding compound required for the pressing and repressing should always be larger than the final molded part so that the mold is completely filled. This guarantees that the molded part is fully formed, and no air is pressed in. The excess molding compound remaining in the prechamber, also referred to as a waste cake, must be removed before the start of the new cycle and replaced by new molding compound. To also process long fibers or fiber semifinished products (prewovens/preforms), these are placed in the mold beforehand and extrusion coated with the molding compound. In addition, the cavity (mold cavity) is usually evacuated to avoid air inclusions.
Resins having a low viscosity are usually used as injection resins. As a result, the flow resistance during flow through the mold remains low, and minor pressure differences are necessary for the filling. Reaction resins for RTM processes are offered as specialized injection resins made of a resin component and a hardener component. In the RTM process, during the injection process resin flows through the mold space at the appropriate flow rate, fills the mold space, wets the inserted materials, and exits from the mold.
Low-reactivity resin systems may be mixed prior to the infusion. If the use of high-reactivity resin systems is desired, the resin and hardener may be mixed only directly in the infusion line or the mold. Lower cycle times are thus possible. Processes in which the injection resin components are mixed only immediately prior to injection are also known as reaction injection molding (RIM) processes.
The patent literature, for example in the publication DE 600 11 752 T2, describes a method for producing structural parts from composite material according to the resin injection process, and a corresponding apparatus. In this approach, the vacuum sealing of the closed mold is verified prior to the resin transfer, and cured resin residues which are not used for the component are removed from a container which is used for additional conditioning and supply of resin. In addition, the pressure is measured to determine the seal-tightness of the mold, and a temperature control device is provided.
Furthermore, a method and a mold for producing fiber composite components are known from the publication DE 10 2007 060 739 A1, in which the progression of the flow front is detected by pressure sensors facing the mold cavity, and the cooling in the edge region is varied as a function of the flow front.
DE 10 2009 010 692 A1 describes, among other things, an apparatus and a method for carrying out an RTM process, the apparatus having an injection unit and a closable mold provided with a mold cavity, and the injection unit being coupled to the mold in such a way that an injection resin is introducible into the mold, the mold having at least one resin outlet which is closable by means of a closing device and connected to the mold cavity, and from which the injection resin may escape after the mold has been filled. The fiber composite component may be monitored during the production process by means of a process sensor system, the process sensor system being coupled to the process actuator in a controlling manner. A camera is provided in a transparent area in the mold, and records an image of the resulting fiber composite component. In addition, the pressure and the temperature in the mold are detected by means of appropriate sensors. Furthermore, a mixing head is provided in which the components are mixed immediately before being injected. In this approach there is no option for recognizing the escape of resin from the mold when it is filled.
An apparatus for producing components from fiber composites is known from the publication DE 19922850 C1, in which a mold is provided which has connecting means for injecting a resin and for reducing pressure and which is composed of cooperating molded parts, at least one molded part having a dimensionally stable design corresponding to the contour of the outer surface of the component, and a fiber structure fitting being insertable between the cooperating molded parts. The bottom part of the mold is the entry device for the resin, for which purpose appropriate channels are provided. A connecting means in the form of a flow valve is provided at the input side and output side of each channel, and is connected to a control system via control lines. An individual line may thus be opened or closed; however, it is also not possible to appropriately detect the escape of resin from the mold.
DE 10 2005 053 691 A1 describes a mold for a resin transfer molding process, having a cavity, a resin trap, and a transition area, the cavity being configured in such a way that a component may be accommodated therein. The resin trap is integrated into the mold, and the transition area is configured in such a way that it may be used to establish a connection between the cavity and the resin trap. A resin tube is provided between the resin trap and the cavity, and is closable by means of a stopcock. In this approach as well, no additional means are present for detecting the escape of resin from the cavity.
An approach for producing reinforced plastic structures is described in the publication GB 944955, in which the mold has a mold cavity, on the outlet side of which a line having a trap for the injected material and the material escaping from the mold is situated. A shutoff valve is situated downstream from the trap.
A similar approach is described in U.S. Pat. No. 5,403,537 A, in which a resin trap and a shutoff valve are provided, also on the outlet side. However, the valve is situated between the mold and the resin trap.
Likewise, a major disadvantage of the two approaches mentioned above is that the escape of resin from the mold is not directly detectable. Thus, the above-mentioned approaches have the drawback that it is not easily and reliably detectable, not even by measuring the pressure rise, if the mold is filled with the injected material and resin has escaped from the mold cavity.
The object of the invention is to develop an apparatus for carrying out a resin transfer molding (RTM) process, and an RTM process for producing in particular integral components, by means of which it may be easily signaled that the mold cavity is completely filled with injection material, and also ensured that material escaping from the mold cavity is supplied for reuse.
This object is achieved by the features of claims 1 and 7. Advantageous embodiments result from the subclaims.
To this end, the apparatus according to the invention has an injection unit and a closable mold provided with a mold cavity, the injection unit being coupled to the mold in such a way that an injection resin is introducible into the mold, and wherein the mold has at least one resin outlet, which is closeable by means of a resin flow closing unit and is connected to the mold cavity, and from which injection resin may escape after the mold has been filled, and a measuring unit which detects the escaping resin flow is situated at or downstream from the mold outlet, and is coupled to the resin flow closing unit in such a way that the resin flow closing unit closes when resin flow is detected by the measuring unit.
The configuration of the measuring unit for detecting the resin flow at the resin outlet or downstream therefrom may be implemented very easily and cost-effectively, since the measuring unit does not have to be integrated directly into the mold.
For the detection of escaping injection resin using the measuring unit, an appropriate signal for closing the resin flow closing unit is output, in particular via a control system, as a result of which the resin flow closing unit is closable, and therefore the further escape of resin from the mold cavity stops.
In particular, the apparatus has a plastic hose adjoining the resin outlet of the mold, through which injection resin escaping from the resin outlet flows, the measuring unit detecting the resin flow through the plastic hose, and for this purpose being designed in particular as a capacitive measuring unit for resin flow detection.
The plastic hose is preferably transparent, at least in places, and the measuring unit is designed in the form of an optical measuring unit/sensor or capacitive measuring unit/sensor for resin flow detection which is situated at the transparent area.
The resin flow closing unit for interrupting the resin flow is in particular a valve, for example a pneumatically controlled check valve.
The plastic hose connected to the resin outlet of the mold leads to a collection container for the injection resin escaping from the mold, which is preferably detachably connected to the hose, for example via a clamp connection.
The collection container is advantageously designed as a disposable collection container which is removable after the injection resin cures and, together with the collected resin, is suppliable to thermal processing/reuse.
The pressure in the mold cavity is detectable by one or more pressure sensors, wherein the resin flow closing unit, which has been closed by the signal of the measuring unit, is reopened when a predefined pressure is exceeded, thus avoiding damage to the mold or degradation of the quality of the component to be produced, on account of excessive pressure.
When the resin flow closing unit is opened upon exceedance of a predefined pressure in the mold cavity, the measuring unit for detecting the resin flow is deactivated to prevent it from once again triggering a signal for closing the resin flow closing unit when additional resin flows through the resin outlet or the line attached thereto for the desired pressure reduction.
After the pressure drops below the predefined maximum pressure, the resin flow closing unit in the form of the valve is once again closed.
The mold has a number of resin outlets, corresponding to the configuration of the integral components, which preferably are each connected at the highest position in the mold cavity. An appropriate measuring device for detecting the resin flow as well as a resin flow closing unit are associated with each resin outlet or the line adjoining same. All the lines may lead into the same collection container, or a separate collection container may also be connected to each line or each tube.
The apparatus also has an injection unit which is coupled to the mold, having a mixing unit (mixing head) for a resin and a hardener for the injection resin, which is connectable to the mold and supplied with the two components to be mixed via two feed hoses from two storage tanks by means of a separate motor-pump in each case, the mixing head having a heating device for heating, and a mixing device for mixing, the two components. It is advantageous if the conveyed volume of the components to be mixed is reduced when a predefined pressure in the mold cavity is exceeded.
In addition, discharge hoses are provided parallel to the feed hoses and return/recirculate the unmixed components to their respective storage tanks.
For the production of integral components, a preform is placed in the mold cavity prior to injection of the injection resin.
The method in the form of the resin transfer molding (RTM) process for producing in particular integral components is carried out using an injection unit and a closable mold, provided with a mold cavity, to which the injection unit may be coupled, an injection resin being introducible into the mold, and the mold having at least one resin outlet which is closable by means of a resin flow closing unit and is connected to the mold cavity, and from which injection resin may escape after the mold has been filled, wherein a measuring unit which detects the escaping resin flow is situated at or downstream from the resin outlet, and is coupled to the resin flow closing unit, and a signal for closing the resin flow closing unit (5) is output to the resin flow closing unit via a control system when resin flow is detected by the measuring unit, and the resin flow closing unit is closed and the resin flow escaping from the mold is interrupted.
The pressure which occurs in the mold cavity during the RTM process is advantageously detected by at least one pressure sensor, and when a predefined pressure is exceeded, the resin flow closing unit, which has been closed by the signal of the measuring unit/sensor, is reopened.
When the resin flow closing unit is opened upon exceedance of a predefined maximum pressure in the mold cavity, the measuring unit for detecting the resin flow is deactivated, and the resin flow closing unit once again closes after the pressure drops below the predefined maximum pressure,
It is also possible to reduce the conveyed volume of the components to be mixed when a predefined pressure in the mold cavity is exceeded.
Different processing modes having different automation levels may be achieved by means of the method.
In a manual/automatic mode, the first closing (of the resin flow closing unit) is triggered as a result of the sensor(s) detecting resin escaping from the resin outlet(s), and at the start of a flushing operation in the further injection process, the opening and closing process of the resin flow closing unit is detected and stored by the control system as a chronological sequence of the on/off (0, 1) state of all connected resin flow closing units until the end of the injection process, the control system outputting auditory and/or visual warnings if the pressure parameters are above the predefined limit values.
In a partially dynamic process, the routine which is detected and recorded in a manual process is carried out as a temporally detected process of the opening and closing of the pneumatic valves of the check valve unit (resin flow closing unit), the routine of the partially dynamic process in an automatic mode being available as a file, and being read by the software upon start-up of the automatic mode, and being started and executed at the appropriate time in the injection process.
It is possible for the automatic process to be carried out also taking into account the changing/increasing pressure in the cavity as the injection process proceeds over time, the control system opening the valves (resin flow closing unit) at which exceedances of the pressure limit values occur, until the values drop below the limit values.
A learning process may thus be implemented from the manual process all the way to the automatic process.
The invention provides a simple and reliable apparatus for the RTM process by means of which it is recognizable that the mold cavity is completely filled, which prevents excessive strain on the mold and the component to be produced as the result of excessive pressure in the mold, and by means of which excess resin escaping from the mold may be supplied for reuse, or in the event of malfunctions, unmixed components present in the system may be returned, for example to their respective storage tanks or other collection containers.